The present invention pertains to radiolocation of mobile vehicles within the coverage of at least one scanning radar of a secondary radar system.
More particularly, the invention concerns a collision avoidance/proximity warning system, capable of determining the slant range and/or relative bearing to mobile vehicles, that is based on secondary radar such as the National Air Traffic Control Radar Beacon System (ATCRBS) and the International Civil Aviation Organization (ICAO) Secondary Surveillance Radar System.
Major airports and way points are presently equipped with secondary surveillance radar (SSR) adapted to cooperate with transponders carried on aircraft to discriminate against interference and ground clutter and to provide for transmission of identification and other data, such as altitude, from the aircraft to the ground-based radar. A traffic controller observing the radar display directs the pilots of the involved aircraft by radio, usually with voice communication, so as to maintain or restore safe separations between aircraft. Such a system is limited in capability because each aircraft must be dealt with individually and requires its share of the controller's time and attention and its share of the available radio spectrum. When traffic is heavy, takeoffs and landings are delayed, and the possibility of collision increases.
The number of mid-air collisions and near misses has become so large in recent times that numerous inter-aircraft cooperative proximity warning systems have been proposed. Those more prominently under study or development at this time involve frequent or quasi-continuous exhange of signals between all cooperative aircraft within the region of interest and make no provision for non-cooperating aircraft. The required airborne equipment would be bulky and expensive, use more of the already crowded radio spectrum and would be generally independent of other needed and existing equipment, such as transponders. Another drawback of some of the proposed systems is that they provide only relative positional information, without ground reference but in effect with respect to a randomly floating reference.
At least some of the disadvantages of these proposed systems may be overcome by providing aircraft with a proximity warning indicator based on the already existing secondary surveillance radar system. Particularly if the range and bearing of all aircraft in the proximity of one's own aircraft can be obtained entirely passively-- i.e., by merely "listening" to the transponder replies of nearby aircraft to interrogations of radar ground stations-- it will be possible to provide an effective warning in time to avoid collisions without major outlay for an entirely new system and without utilizing an additional portion of the radio spectrum.